1. Field of the Invention
The present invention relates generally to former boards and, in particular, to former boards for use with a moving web in a printing press.
2. State of the Art
Former boards are used in printing presses to, for example, fold a continuous moving web. Former boards are typically shaped as inverted triangles over which the moving web is transported. A typical former board introduces air into an interface between the web and the former board through radial holes drilled into forming bars over which the web travels.
FIG. 1 shows an exemplary former board 100 having a cylindrical shape. FIG. 2 shows an exemplary former board 200 having a conical shape.
In the FIG. 1 example, a traveling web moves over a roll 102 from a wider portion 104 of the former board 100 toward a lower portion 106 of the former board, at which location the web has been formed into a folded web at or near its previous centerline. Air is introduced through spaced holes 108 along lengths of former board bars 110, 111 to create a cushion of air between side surfaces 112, 114 of the former board and the web. Similarly, the conical former board 200 of FIG. 2 includes holes formed along a length of former board bars 202, 204 to create a cushion of air between the side surfaces and the moving web.
However, the former boards illustrated in FIGS. 1 and 2 do not allow for even pressure distribution in areas between the holes. To the contrary, low pressure areas result which allow the paper to contact the stationary forming bars. This contact results in marking of the web. Another consequence of the web contacting the forming bars is that the bars wear over time.
In addition to low pressure areas being formed at locations on the former board between the air holes, low pressure areas can also occur at the locations of the air holes themselves when the air flow is not properly adjusted. Air flow through individual holes in the forming bars of FIGS. 1 and 2 are not separately controlled. The flow of air through the various holes of the forming bars is collectively controlled using, for example, a metering valve. However, the metering valve provides a very coarse adjustment of a pressure pad established between the web and the former board. These low pressure problems can lead to the air holes being plugged with ink or paper dust, thereby blocking passage of air. This in turn leads to former board wear and damage to the web.
Air flow has been used with commercially available air turn devices, wherein a cushion of air is created to change a web's direction of movement without contacting the web. This type of device is used when the ink applied to the web is still wet, such that any contact between the web and an air turn would damage and likely halt the printing process. The air turn device requires a relatively large air gap between the web and the air turn. However, as those skilled in the art would appreciate, a former board requires a minimum air gap to achieve acceptable product quality.
Attempts to address the foregoing deficiencies in former boards have resulted in repositioning of the air holes on the forming bars and/or increasing the number of air holes. In addition, to minimize former board wear, low friction tape has been applied to the forming bar surface. However, these solutions have not eliminated the foregoing deficiencies, or rendered the former board suitable for introducing a fold into a moving web. Accordingly, it would be desirable to develop a method and apparatus for implementing a former board which addresses the foregoing deficiencies.